Even if you are the sole developer, source control still is invaluable:

It lets you keep track of all your changes, giving you the confidence to experiment with big changes because you know you can just roll back. This is much better than copying your entire project each time you do something big.

It lets you view the entire revision history of your code

There are tons of free helper-tools, like NUnit for unit tests, FxCop for static code analysis, and more. Scott Hanselman does a great job of summarizing these.

Given that the knowledge is free (via millions of internet tutorials, blogs, and reference guides), and the tools are free, all you need is a computer and motivation, and even the young hobbyist can become a great developer.